1. Field of the Invention
This invention relates generally to broadband microwave power dividers suitable for distributing an input signal between a selected number of output loads.
2. Description of Prior Art
Microwave power dividers are useful in a wide variety of instrumentation and system applications, such as feeding signals to multiple antennas. Power dividers can also be used to combine microwave signals by applying the signals to be combined to what would normally be considered the outputs of the divider. Combining signals in this manner is a popular way of obtaining higher output power from semiconductor signal sources.
Thus, for ease of discussion, the invention is described as a power divider. As is commonly understood, the term also means power combiner, in which case the current direction is simply reversed, with each output functioning as an input and each input functioning as an output.
Early N-way power dividers with equal power division are described in U.S. Pat. No. 3,091,743 issued to Wilkinson; and 4,129,839 issued to Galani et al. These power dividers have the disadvantage of covering a relatively narrow frequency bandwidth--typically under 1.5:1. A two-way equal division power divider capable of greater bandwidth is described by Cohn in "A Class of Broadband Three-Port TEM-Mode Hybrids," IEEE Transactions on Microwave Theory and Techniques, vol. MTT-16, no. 2, February 1968, pp. 110-116. A schematic diagram of the power divider described by Cohn is shown in FIG. 1. Most of the commercially available broadband two-way equal division power dividers are made according to Cohn's design. An extension of the Cohn design to an N-way equal division power divider is described by Yee et al. in "N-Way TEM-Mode Broad-Band Power Dividers," IEEE Transactions on Microwave Theory and Techniques, vol. MTT-18, no. 10, October 1970, pp. 682-688. A schematic diagram of the power divider described by Yee et al. is shown in FIG. 2. The devices of Cohn and Yee et al. both obtain increased bandwidth by using cascaded equal length transmission-line segments with interconnecting resistors.
Both of these devices have a band pass type of frequency response and have the disadvantage of having the frequency range limited by the pass band. Another serious disadvantage is that, for a wide bandwidth, a large number of resistors are required. Another disadvantage of these devices is that they require interconnecting resistors with values that are relatively high with respect to the Z.sub.o characteristic impedance of their input and output ports. For example, a 10:1 bandwidth 2-way equal division power divider described by Cohn uses (assuming 50 ohms input and output impedances) 7 resistors varying in value from 129.6 to 616.1 ohms. These high values of resistances typically have parasitic reactances that significantly degrade performance at higher (above 10 GHz) microwave frequencies. The Yee et al. device requires similar high resistance values.
Conventional power dividers require relatively large numbers of resistors. For example the 10:1 bandwidth, 2-way equal division power divider described by Cohn uses 7 transmission line segments and 7 resistors. A 2:1 bandwidth, 3-way equal division power according to Yee et al. would typically require 6 resistors. It is desirable to construct power dividers in microstrip. Microstrip is desirable because of relatively low cost and compatibility with integration into microstrip systems. A widely used microstrip device that gives equal power division and outputs 90.degree. apart is described by Lange in the paper "Interdigitated Stripline Quadrature Hybrid, " IEEE Transactions on Microwave Theory and Techniques, vol. MTT-17, December 1969, pp. 1150-1151. The Lange device typically has poor isolation above 25 GHz because of discontinuities introduced by bond wires and abrupt junctions at the ends of the coupled lines.